During the creation and subsequent operation of a subterranean well, the operator may wish to perform acts that could potentially damage the underground formations and their ability to produce desirable formation fluids. For example, the operator may wish to inject water into the well. The operator may do this to enhance the productivity of a well or to dispose of waste water.
In addition to water, other fluids are routinely used in the operation of a subterranean well. Drilling fluids are used to aid in the drilling of a well; both to cool the drill bit and to remove drill cuttings from the well. Completion fluids are used when completion operations are performed in a producing formation. Workover fluids are used when working over previously completed wells. In all of these cases, it can be desirable to seal off producing formations in order to prevent fluid loss from the well to the formation and to prevent possible damage to the formation.
One way of protecting the formation is by forming a filter cake on the surface of the subterranean formation. Filter cakes are formed when particles suspended in a wellbore fluid coat or plug the pores in the subterranean formation such that fluid is substantially prevented from passing between the formation and the wellbore and vice versa. A number of ways of forming filter cakes are known in the art, including the use of both clay and non-clay based drilling fluids.
In addition to the intentional formation of filter cakes, filter cakes can also be produced unintentionally. For instance, when drilling the well, the particles contained in the drilling mud can lodge in the pores of a formation that the operator desires to bring into production.
Whether the formation of the filter cake was unintentional and intentional, it is desirable to remove the filter cake when the formation is brought into production. The presence of the filter cake can hinder the passage of fluid from the formation to the wellbore and thereby retard production rates.
Various ways have been developed by those skilled in the art to form filter cakes that can be easily removed. For example U.S. Pat. No. 5,251,697 discloses the addition of calcium carbonate to water being injected into a well. The calcium carbonate particles either clog the pores in the subterranean rock formations or collect and build a filter cake. When the filter cake is to be removed, the '697 patent directs the operator to circulate an acid wash, preferably hydrochloric acid, into the well. The acid wash will dissolve the calcium carbonate and thereby destroy the filter cake. At this point the well can be brought into production or additional work performed on the well.
In U.S. Pat. No. 5,607,905 is disclosed a method of forming a filter cake using a fluid containing polysaccharide polymers, bridging particles and an alkaline earth metal or zinc peroxide. The method of the '905 patent seeks to have to particles in the fluid bridge over the formation pores rather than plug the holes. The polysaccharide polymers are typically added to the wellbore fluid as viscosifiers or fluid loss control additives. An earlier patent, U.S. Pat. No. 5,238,065; taught that filter cakes containing polysaccharide polymers could be removed by contacting the filter cake with a brine fluid containing a zinc or alkaline earth metal peroxide, an acidic substance such that the pH of the solution was between 1 and 8 and an activator for the peroxide. A wash solution would then be used to remove the filter cake. The degrading substance incorporated within the filter cake is generally referred to as an internal breaker. The use of an internal breaker was beneficial because it required less peroxide, less loss of wash fluid to the formation, and gave more complete removal of the filter cake.
However, these prior art systems contain several drawbacks. First, the peroxides used have a tendency to emit peroxide prior to activation, resulting in premature weakening of the filter cake as well as reducing the effectiveness of the polymers used to control fluid loss and viscosity. Additionally, these prior art methods require action on the part of the operator to remove the filter cake. This results in additional cost and delay. Also, the acid or other solutions used to dissolve filter cakes can have a harmful effect on the formation. The present invention provides an improved method for removing filter cakes in a subterranean well and the formulation and use of drilling fluids used to form such filter cakes.